Modulation of the electronic communication between two equivalent ferrocene centers by proton transfer, solvent effects and structural modifications
Introduction
Electronic communication effects between equivalent redox centers have been the subject of extensive research work and led to numerous reports in the chemical literature.1 Due to their synthetic accessibility, multinuclear metallocene compounds have played an important role in this area of research.2 Recently, we reported a new type of dinuclear ferrocene compound containing a CH2–N(R)–CH2 tether.3 These compounds display a moderate degree of electronic communication between their two identical ferrocene residues, which essentially disappears upon protonation or methylation of the central tertiary nitrogen in the tether.3 In order to better understand this finding, we have prepared a series of three compounds having aliphatic R groups with variable degrees of steric bulk (compounds 1, Chart 1, Scheme 1 and 3; see Chart 1 for structures). We have also prepared another three compounds (4, 5, and 6; Chart 1) having aromatic R groups to investigate the influence of electron withdrawing/releasing effects on the communication between the ferrocene centers.
In spite of the abundance of dinuclear ferrocene compounds that are already described in the literature,2 the compounds reported here are attractive due to the following reasons: (i) their ease of preparation, (ii) the presence of a centrally located nitrogen atom in the tether connecting the two ferrocene groups, and (iii) the possibility to manipulate (reversibly or irreversibly) the extent of electronic communication between the ferrocene centers using the reactivity of the tertiary amine functional group. In this paper, we provide a detailed account of the synthesis and electrochemical properties of these compounds, as well as relevant 1H NMR spectroscopic and single crystal X-ray diffraction data.
Section snippets
Synthesis
The preparation of the dinuclear ferrocene compounds Chart 1, Scheme 1 is based on the known dissociation in polar solvents of the (ferrocenylmethyl)trimethylammonium ion to produce trimethylamine and the corresponding ferrocenylmethylene carbocation.4 In the presence of nucleophiles, this carbocation gives rise to ferrocenylmethylated products.4, 5 Therefore, reaction with a primary amine easily yields the corresponding bis(ferrocenylmethyl) tertiary amine (see Scheme 1) plus residual amounts
Materials
All solvents and chemicals for synthesis were commercially available and were used without any further purification. Solvents were removed on a rotary evaporator connected to a water-aspirator and the remaining traces were evaporated on a vacuum oven typically set at 70 °C overnight. For electrochemistry, acetonitrile and dichloromethane (99.9%, HPLC grade) were purchased from Aldrich.
Methods and instrumentation
1H NMR spectra were recorded on a Varian VXR-400 spectrometer and chemical shifts were measured with reference
Conclusions
In the solution phase the dinuclear ferrocene compounds 1 and Chart 1, Scheme 1 display different degrees of electronic communication that correlate well with geometric parameters describing the average distance between their ferrocene centers in the solid state. Although we could not determine the crystal structure of 3, the unbranched nature of its hexyl substituent fits very well with the trend that extrapolates from the relative steric bulk of the substituents in compounds 1 (t-butyl) and
Supporting Information
Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as CCDC 155780–155784 for compounds 1, Chart 1, Scheme 1, 3+ 6 and 5, respectively. Tables of atomic coordinates, bond lengths and angles, and anisotropic displacement parameters for these five compounds (Tables S1–S15).
Acknowledgements
The authors are grateful to the NSF (to AEK, CHE-9982104) for the generous support of this research work. T.R. acknowledges the CCD diffractometer fund from the University of Miami. The authors also wish to thank Ms. Eden Pacsial for assistance in the collection of the X-ray data set for 3+.
References (16)
- et al.
Adv. Inorg. Chem. Radiochem.
(1967)Angew. Chem.
(1984)Chem. Soc. Rev.
(1995)Acc. Chem. Res.
(1997)Chem. Eur. J.
(2000)et al.Acc. Chem. Res.
(2000) - et al.
Chem. Rev.
(1997)et al.Chem. Rev.
(1999) - et al.
Organometallics
(1999) - et al.
Tetrahedron Lett.
(1972) - et al.
Tetrahedron Lett.
(1979) - Beer, P. D.; Nation, J. E.; McWhinnie, S. L.. W.; Harman, M. E.; Hurtsthouse, M. B.; Ogden, M. I.; White, A. H. J....
- et al.
Electrochemical Methods: Fundamentals and Applications
(1980) - et al.
Electrochemical Methods: Fundamentals and Applications
(1980)